In-field rice straw management

Open-field burning

Figure 1. Rice straw burning in an open field in Vietnam, 2013Open-field burning of rice straw (Figure 1) effectively removes large volumes of biomass and helps control weeds and a variety of pests and diseases (Ponnamperuma 1984). However, research indicates that the advantages of burning are offset by the disadvantages, including nutrient loss, depletion of soil organic matter (SOM), and reduction in the presence of beneficial soil biota (Mandal et al 2004). Rice straw burned in the field also causes greenhouse gas emissions (GHGE), including 0.7–4.1 g of CH4 and 0.019–0.057 g of N2O per kg of dry rice straw, and emission of other gaseous pollutants such as SO2, NOx, HCl and, to some extent, dioxins and furans (Oanh et al 2011, Jenkins et al 2003). Rice straw burning is also an important source of aerosol particles such as coarse dust particles (PM10) and fine particles (PM2.5) (Chang et al 2013), affecting regional air quality and the radiation budget of the earth (Engling et al 2009). For these reasons and the unaffordable health risks that rice straw burning poses on the environment, open-field straw burning has been banned or is strictly regulated in agricultural regions across the world.

Incorporation

Figure 2. A Yanmar machine combining harvesting, chopping rice straw, and spraying Trichoderma into chopped straw for faster decomposition.Incorporation of rice straw into paddy soil is another strategy to manage rice straw, but doing it improperly and ineffectively can result in a decrease in production efficiency (Mandal et al 2004, Yadvinder-Singh et al 2005, and Dobermann and Fairhurst 2002) and an increase in greenhouse gas emissions (GHGE) (Sander et al 2014). Some farmers, however, do not practice incorporation because of the slow decomposition rate of rice straw, which may not be completed within the short turnaround time of less than three weeks before the next rice cropping season. Our recent research at IRRI showed that the total carbon dioxide equivalent (CO2-eq) per ha converted from CH4 and N2O in a rice crop season with straw incorporation emitted about 3,500 kg CO2-eq per ha. This amount of GHGE was almost 1.5 times higher than the amount emitted from the practice of rice straw removal.

This has led researchers to conduct studies on how to speed up the decomposition rate of rice straw, one of which investigated the use of fungal inoculums (Goyal and Sindhu 2011, Ngo et al 2012). Adopting this technology, a machine was introduced recently in Vietnam, which combines three functions of combine harvesting, chopping rice straw, and spraying inoculums into chopped straw to foster its decomposition in the soil (Figure 2).